Saturday 25 October 2008 - Filed under Journal
So I’ve been building model rockets for the last month or two. It started when I told Myles how I used to launch rockets when I was a kid. He wanted to do it, too. So we went to the hobby store and bought a ready-to-fly (RTF) rocket, a launch pad and a kit rocket we had to make ourselves.
We launched the RTF rocket and had fun with it. Meanwhile, we worked on the kit rocket, which was an Estes Hi-Flyer. We got it done eventually and then went to a launch held by Tripoli Minnesota. The launch was a lot of fun, with a lot of big and cool rockets. We signed up to launch the Hi-Flyer and after it got off the pad it went crazy, flying in loop-de-loops and crashing on the ground amidst the spectators. We laughed it off but I was embarrassed. It was one of those learning moments, though. I’ll tell you what was wrong in a minute. (Any rocketeers reading this know exactly what was wrong.)
But I had the bug and ordered a LOC Aura. It’s a bigger rocket, about 2 feet tall, and capable of flying larger motors than are made by Estes. Let me make a confession — I’m a “quick and dirty” guy. My memories of making models when I was a kid are that I always screwed it up. Literally every time I made a mess of it. In astronomy, I am kind of the same way when it comes to the mechanical things. I am very careful and systematic when I handle the data, but I’m just not very good at delicate, detail-oriented things.
So I enjoyed the challenge of taking my time, a few minutes every day, making the Aura to the best of my abilities. I sanded and glued and sanded and primed and painted. Then I’d sand more and paint more. I tried to make it perfect. In the end it wasn’t perfect but it was very close, as close as I’ve ever been, that’s for sure.
In the meantime I wanted to know what went wrong with the Hi-Flyer. Actually, on the drive home from the failed launch, I read through the instructions I had with me and noticed a major oversight on my part — always test the stability!
With a rocket, the fins act as wings but unlike wings on a plane, the goal is not to provide lift but to keep the rocket going straight. Each of the 3 fins, if displaced from equilibrium, tries to bring the rocket back in line with the wind. It’s basically like a wind vane. But that’s not all, the fins cause the rocket to pivot on its center of gravity when they correct the flight. The center of gravity is the balance point of the rocket with respect to gravity. There is a similar point, called the center of pressure, which is the balance point, in a sense, with respect to the air pressure. If the center of gravity is between the center of pressure and the nose of the rocket, the rocket is stable and it will fly into the wind. If the center of gravity is on the same point as the center of pressure, the rocket is stable but barely so and it wanders around a bit. If the center of gravity is between the center of pressure and the tail of the rocket, the rocket is called “negatively stable” and it flies around in loop-de-loops and crashes to the ground.
You can calculate the center of pressure and you can easily find the center of gravity by balancing the rocket on your finger. You should never fly a rocket that is negatively stable.
So I learned all this, mainly by reading Handbook of Model Rocketry. So while it was great fun to make the rocket physically, it was also a lot of fun to learn about the aerodynamics and math behind how rockets fly.
So the Aura was not only beautiful but it was slightly over-stable. If the center of gravity is forward of the center of pressure by the diameter of the rocket, it’s called “1 caliber” stability. The Aura was at 2 caliber stability or so, which gives a nice margin of error.
So today we got out and launched both the Hi-Flyer and the Aura. It was really fun. We flew the Hi-Flyer on a “A” engine first, just to make sure it was stable. I had poured Elmer’s glue in the nose cone to bring the center of gravity forward. It flew great and landed right back in the field. Then we flew the Aura on an “F” engine. It went completely out of site. It arced into the wind and we lost it but then we saw the ejection charge go off. It had a large, blaze orange streamer so we could see it coming down. It landed right next to Dale and Marth’s barn, probably a half mile from where we launched it. I think we probably got to 3000-4000 ft. The rocket was completely unharmed when we finally found it — just a little mud on one fin.
Then we launched the Hi-Flyer again on a “C” engine. I still don’t know what happened. It went up fine and we saw the ejection charge go off but we never saw the rocket again after that. The shock cord might have broke, sending the nose cone flying off on it’s own. It’s a tiny rocket, so if the streamer didn’t come out it could easily be missed. Obviously, since we missed it!
Then we launched the Aura again. The wind had come up and I was worried it was going to blow into the woods to the east. So I pointed it a little bit into the wind. It went off like a rocket and, since the wind was blowing harder, arced even harder into the wind. It almost seems like it was flying horizontal. It went out of sight again and again we saw the ejection charge and the streamer. It looked like it was going to land right back near the launching pad. But the wind didn’t blow it as much as we hoped and the rocket ended up falling into the woods to the west of the field. And it’s a huge field, let me tell you.
We looked for the Aura for probably an hour or more and finally I found it high up in fairly skinny tree. There was no way to get it unless we cut down the tree. Because it wasn’t our tree, we left it there. It’s still there right now. WWAAAAAAAAAAAAAHHHHHHHHHHH.
I am actually a little bit bummed, but mostly it was just a lot of fun. Rockets like the Aura can break the sound barrier and go supersonic. They can go up thousands of feet. It’s a very fun thing to do if you have kids and even if you don’t. Play around. Have some fun. Build some stuff. Blast off.
2008-10-25 » lolife